1. Field of the Invention
The invention relates generally to cellular telephones and more particularly to a Global System for Mobile telecommunications cellular telephone and global positioning system (GSM/GPS) receiver combination using a single chip RF integrated circuit having an efficient divider scheme for using either of a GSM standard reference frequency or a second reference frequency for downconverting a GPS signal.
2. Description of the Prior Art
Global Positioning System (GPS) receivers determine location, velocity, and time by receiving and processing information in GPS signals received from GPS satellites that have been placed in orbit around the Earth by the United States Government. The GPS signal from each satellite carries data for the location-in-space of the satellite and time-of-transmission on carrier frequencies that are the same for all the satellites. The data from each satellite is spread with a pseudo-random noise (prn) code that is distinct for that satellite. A GPS receiver uses the distinct prn code for distinguishing the GPS signals from typically at least four satellites and then finds its own location, velocity, and time by solving simultaneous equations using the relative times that the signal from each of the satellites arrives at the receiver and the locations-in-space and times-of-transmission from the satellites.
There is a continuous need for improving the performance of GPS receivers in terms of accuracy, acquisition and tracking of low level signals, acquisition time, and immunity to interference. And, there is a continuous need for reducing the size, power consumption, and cost of the GPS receiver. One of the major components in determining the performance, size, power, and cost in the GPS receiver is the radio frequency (RF) circuitry at the front end for downconverting the GPS satellite carrier frequency to an intermediate or baseband frequency. A second major component is the digital signal processor (DSP) circuitry for processing the signal at the intermediate or baseband frequency for providing GPS signal correlations. Due to the relatively high frequency of the GPS satellite signal, most GPS receivers until recently have used discrete components for the front end RF circuitry. Although the performance of such RF circuitry may be very good, these discrete components represent a large portion of the size, power, and cost of a modern GPS receiver. Gallium Arsenide (GaAs) integrated circuits (IC)s have been developed using field effect transistors (FET)s as active devices for replacing the majority of the discrete components in the RF circuitry. Unfortunately, GaAs ICs have been and continue to be relatively expensive because the commercial manufacturing volume of GaAs ICs is low and because the GaAs material is more expensive than Silicon and the processing steps in manufacturing a GaAs IC are relatively difficult. Silicon bipolar ICs using bipolar transistors as active devices have been used for the front end RF circuitry in the GPS receiver. Such ICs can be more difficult to design because the frequency response, noise figure, and power consumption for the silicon IC bipolar transistors are typically not as good as for the GaAs IC FETs. However, the silicon bipolar ICs are less costly and have recently been shown to have sufficient performance for most applications. There continues to be a need for improvements in RF ICs in order to improve performance and reduce size, power consumption, and cost in a GPS receiver.
Cellular telephones are commonly used together with GPS receivers for communicating the location of the receiver. In most of these uses the location is read by a human user from a stand alone GPS receiver and then spoken into a stand alone cellular telephone. In an improvement over this system, the location is communicated electronically from the GPS receiver to the cellular telephone using a digital interface. Recently, GPS receivers and cellular telephones have been packaged together for reducing their combined cost and size by sharing the same housing and power supply. However, in existing systems where a GPS receiver and a cellular telephone coexist in the same package they share little functionality and little or nothing has been done to take advantage of the circuitry and functions of the cellular telephone for improving the performance and reducing the cost of the GPS receiver. There is a need for a GPS receiver and cellular telephone combination that uses circuitry of the cellular telephone for improving the performance and reducing the cost of the GPS receiver.